A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g., including part of, one or several dies) on a substrate (e.g., a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the beam of radiation in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
Photolithography is widely recognized as one of the key steps in the manufacture of ICs. At present, no alternative technology seems to provide the desired pattern architecture with similar accuracy, speed, and economic productivity. However, as the dimensions of ICs and/or other devices made using photolithography become smaller, photolithography is becoming one of the most, if not the most, critical gating factors for enabling miniature IC or other structures to be manufactured on a truly massive scale.
A theoretical estimate of the limits of pattern printing can be given by the Rayleigh criterion for resolution as shown in equation (1):
                    CD        =                              k            1            *                    ⁢                      λ                          NA              PS                                                          (        1        )            where λ is the wavelength of the radiation used, NAPS is the numerical aperture of the projection system used to print the pattern, k1 is a process dependent adjustment factor, also called the Rayleigh constant, and CD is the critical dimension, i.e. the smallest space between two features of a pattern (such as, for example, lines or contacts), permitted in the fabrication of a device layer and/or the smallest width of a line or any other feature. In the context of an array of features characterized by a certain pitch at which the features are spaced in the array, the dimension CD in Equation 1 represents the value of half of a minimum pitch that can be printed lithographically, referred to hereinafter as the “half-pitch”.
It follows from equation (1) that a reduction of the minimum printable size of features can be obtained in three ways: by shortening the exposure wavelength λ, by increasing the numerical aperture NAPS or by decreasing the value of k1.
Current resolution enhancement techniques that have been extensively used in lithography to lower the Rayleigh constant k1, thereby improving the pattern resolution, include the use of phase shift masks and the use of off-axis illumination. These resolution enhancement techniques are of particular importance for lithographic printing and processing of contact holes or vias which define connections between wiring levels in an IC device, because contact holes have, compared to any other IC features, a relatively small area. Contact holes may for example be printed using conventional on-axis illumination in combination with a dark-field alternating-aperture phase shift mask, and further using positive resist. With such an arrangement, only the plus and minus first order diffracted beams emanating from a dense pattern of contact holes on the reticle are capable of traversing the projection system pupil to contribute to imaging, resulting in an enhanced depth of focus. When compared to using on-axis illumination in combination with a dark-field binary mask (with transmissive holes in a chrome layer to pattern the radiation beam) an improved resolution is obtained as well.
Alternatively, contact holes may for example be printed using off-axis illumination in combination with either a dark field binary mask or a dark field 6% attenuated phase shift mask, in combination with the use of positive resist. Here the off-axis illumination improves resolution and depth of focus in a similar way, whereby only one first order diffracted beam and the zeroth order beam emanating from the reticle pattern traverse the projection system pupil to contribute to imaging. One of the imaging quality parameters of relevance for high resolution lithography is the Mask Error Enhancement Factor, referred to by MEEF. Errors in the size of features of the mask pattern may appear enhanced by the factor MEEF in the projected image at wafer level. In particular the imaging of contact holes by means of dark field masks such as described above features a relatively large MEEF, which may become out of tolerance when pushing lithography to the processing of features with ever smaller critical dimension CD. At present, the use of attenuated phase shift masks or binary masks with off axis illumination may not be feasible for patterning contact holes below about 85 nm (at λ=193 nm, NAPS=0.93, and k1=0.4). The techniques mentioned above, based on the use of positive resist, therefore have limited capabilities and may not provide sufficient process latitude (i.e. the combined usable depth of focus and allowable variance of exposure dose for a given tolerance in the critical dimension) for printing half-pitches below a CD obtainable when operating at k1=0.4.
An alternative solution that was recently proposed to print half pitches in the regime below k1=0.4 with sufficient process latitude is to use a vortex mask. (See Mark D. Levenson et al., “The Vortex Mask: Making 80 nm Contacts with a Twist!,” 22nd Annual BACUS Symposium on Photomask Technology, Proceedings of SPIE Vol. 4889 (2002)). A vortex mask is composed of rectangles with phases of 0 degrees, 90 degrees, 180 degrees and 270 degrees. The walls of the phase trenches are nearly vertical, with all four-phase regions meeting at sharp corners, which define the phase singularities. Because the phase of the wave front is not defined at the corner where the rectangles with the four different phases meet, the intensity at that point is necessarily equal to zero in accordance with the laws of physics. In other words, the central core of the vortex must be dark. Therefore, after traversing the vortex mask, the radiation wavefront spirals like a vortex and has a zero amplitude at its central core, instead of forming a plane or a sphere. In combination with a negative resist process, the central axis dark spot of the optical vortex transferred onto the substrate can potentially support larger process windows at small k1 (based on half-pitch) than conventional methods and can allow for smaller holes to be printed with acceptable process latitude. However, a successful implementation of this technology will need the development of appropriate negative-resist tone processes which may be complicated and costly.